test: add coverage reporting with regression thresholds

Add v8 coverage via a test:coverage script and vitest config: measure src
only, exclude the types-only deps.ts and the bin entrypoint main.ts, and
gate on ratcheted thresholds (statements 85, branches 78, functions 90,
lines 85) set just under current coverage. CI runs the unit+integration
tier through coverage so a real drop fails the build.

.gitea/workflows/ci.yml

@@ -47,8 +47,8 @@ jobs:
       - name: Typecheck
         run: npm run typecheck
 
-      - name: Unit and integration tiers
-        run: npm test
+      - name: Unit and integration tiers (with coverage thresholds)
+        run: npm run test:coverage
 
       - name: End-to-end tier
         run: npm run test:e2e